Diabetes is one of the leading causes of morbidity and mortality in the veteran population. A failure to increase the ?-cell proliferation and functional ?-cell mass in response to increasing metabolic demand from insulin resistance associated with obesity and aging underlies most causes of adult onset diabetes in veterans. It is, therefore, imperative to better understand the regulation of ?-cell proliferation, so that it may be targeted for therapeutic purposes to cure diabetes. While Tead1, a critical transcriptional effector of the mammalian Hippo pathway, has been shown to increase cellular proliferation in many tissues, its role in ?-cell proliferation is unknown. In preliminary studies, we demonstrate that Tead1 is robustly expressed in mouse and human islets and has a non-redundant role in regulating ?-cell proliferation. While Yap and Taz, co-activators of Tead1, translocate to the nucleus in hyperplastic islets, ?-cell specific Tead1 deletion leads to diabetes and glucose intolerance, secondary to a loss of glucose stimulated insulin secretion in vivo. Tead1-null islets display a decrease in expression of mature ?-cell markers and a surprising increase in ?-cell proliferation. This increase in ?-cell proliferation occurs even in ?-cells of aged mice that are normally quiescent in controls. Global transcriptome analysis from Tead1-null islets reveals a significant differential expression of genes that regulate cell cycle with a significant decrease in expression of p16 (Ink4a/Cdkn2a), a critical cell cycle inhibitor. Tead1 also binds directly to the promoter of p16, suggesting it may e a direct transcriptional target of Tead1 in ?- cells and mediates the Tead1 regulation of cell cycle. In addition, Tead1 also occupies the cis-promoter elements of critical ?-cell genes, including Pdx1, MafA and Nkx6.1, while the expression of these genes is decreased in Tead1-null islets, suggesting that Tead1 may directly regulate ?-cell function. Hence, we hypothesize that Tead1 regulates the switch between quiescence and proliferation and in maintaining the mature ?-cell phenotype to exert critical control of ?-cell functional mass. The broad goal is to delineate key pathways regulating ?-cell proliferation through genetic loss- and gain-of-function studies using in vivo mouse models and ex vivo mouse and human islets. We will specifically 1. Test if Tead1 regulates adult ?-cell proliferation and quiescence and dissect underlying mechanisms by assessing if Tead1 regulates aging-associated quiescence and decline in ?-cell proliferation and dissect underlying mechanisms. We will test our hypothesis that p16 is a direct target of Tead1 mediating the age-associated proliferative senescence in ?-cells. 2. Test if Tead1 is required for ?-cell function and identify its molecular targets in ?-cells by assessing changes in Tead1-deficient ?-cells in genes regulating function, such as Pdx1, MafA and Nkx6.1, along with identification of direct transcriptional targets of Tead1 that regulate ?-cell function. We will also test if de-differentiation plays a role in the loss of ?-cell function with Tead1 deletion. We will test if Tead1 co-activators, Yap/Taz and its co-repressor, Vgll4, regulate ?-cell proliferation and function. 3. Determine the role of Tead1 and the mammalian Hippo pathway in human islet proliferation and function by assessing if Tead1 regulates ?-cell proliferation in isolated human islets and if this regulatory role is impaired in diabetes. Collectively the proposed studies will critically address how Tead1 regulates ?-cell proliferation and identify novel pathways regulating cell cycle in ?-cells. We envision that the results from ths study will lead to discovery of targeted therapies to modulate ?-cell proliferation to preserve an restore functional ?-cell mass in combating diabetes. With the significant rise in incidence of diabetes in veterans, there is an urgent need to develop novel therapies to reverse the decline in functional ?-cell mass and successful completion of the proposed experiments will address this critical need.